Abstract:

Light quality, i.e. the solar radiation spectrum, is involved in developmental processes of plants,
including trees. Characterisation of morphogenetically active radiation (MAR) within a canopy is necessary
in order to take into account photoregulation of the architecture in tree simulation models. This study
was a first attempt at describing and simulating the spatial distribution of light quality within a walnut
tree crown (Juglans regia L.) using both spectral measurements and a radiation transfer model based on the
turbid medium analogy. Both measurements and simulation were qualitatively in agreement. They showed large
differences in light quality between shaded and sunlit areas. The range of measured and simulated values
was in agreement with values reported in the literature. The quantitative comparison between measurements
and model outputs showed large discrepancies. The latter were attributed to the rough treatment of
scattering in the model, the small amount of punctual measurements made in the tree, and the high
sensitivity of input parameters such as the diffuse to incident radiation ratio and canopy structure
description. Nevertheless the model was mostly able to describe the range of MAR values (phytochrome
equilibrium
,
blue transmittance) found in the tree canopy.